1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a display device, and in particular but not exclusively, relate to a liquid crystal on silicon (LCOS) display device having pixel arrays on a silicon wafer.
2. Description of the Related Art
The age of multimedia has arrived, and a user's demand for large-sized display devices is appreciably increasing, which accelerates the development of the large-sized display devices. Existing cathode ray tube (CRT) projectors have limitations in regard to resolution and luminance. This shortcoming has been offset by recently developed liquid crystal display (LCD) projectors or digital light processor (DLP) projectors.
A display device including liquid crystal on silicon (LCOS) has also been recently proposed, providing higher sharpness, resolution and luminance compared to the CRT, and thus, providing improved overall image quality.
The LCD has a light-transmissive structure in which the liquid crystal is injected between patterned glass substrates and sealed, allowing light emitted from a lamp to pass through the glass substrates. On the other hand, the LCOS has a light-reflective structure in which pixel electrodes are arranged on a silicon substrate, causing light emitted from a lamp to reflect. The transmissive type LCD requires address lines to drive pixel electrodes, which causes the light transmission to be reduced. Also, a grid pattern appears in the LCD, causing the image quality deterioration. The reflective type LCOS can realize a system offering good brightness because pixel driving electrodes do not block the light and therefore increase the light transmission. Also, the LCOS display device can realize a high resolution because it has pixels on a silicon substrate using a semiconductor fabrication process, and advantageously has a high production yield and is also suitable for mass-production. Since the utilization of the semiconductor fabrication process does not require additional investments for production facilities, the LCOS display device has a competitive price.
One of the important parameters for the prevention of image quality deterioration in the display devices using the liquid crystals such as LCD and LCOS display devices is the uniformity of cell gap between two substrates, where the liquid crystals is located. It is important to keep the cell gap uniform because a response speed (converting speed from black to white or from white to black), a contrast, a visual angle, and a luminance of the characteristics of liquid crystal cells have a close relationship with the thickness of the liquid crystal layer. If there is non-uniformity, then the transmittance of light that penetrates through the cell gap loses consistency, causing non-uniform luminance. A solution to this problem is becoming more important because the display devices using the liquid crystals are in the process of shifting from a compact size to a large-size. It has been attempted to solve this problem by the placement of spacers, each having a predetermined size, on a screen unit.
FIG. 1 is a cross-section view of an LCOS display device having a uniform cell gap by the presence of spacers. The LCOS display device comprises a wafer 10, a glass 20, walls 22 oppositely disposed between the wafer 10 and the glass 20 and containing therein a sealant 23 for protecting the liquid crystals, and spacers 25 placed on a screen unit where pixels exist to maintain a uniform cell gap.
The spacer 25 is an important factor in the above structure to keep the cell gap uniform. In the absence of the spacers 25 as shown in FIGS. 2A and 2B, the cell gap between the wafer 10 and the glass 20 may irregularly change. The reason for the deformation of the cell gap is a difference in thermal expansion rates between the glass 20 and the wafer 10 by heat generated during the bonding process thereof. That is, when the temperature decreases to a room temperature after the bonding process at a high temperature, the glass 20 and the wafer 10 contract differently so that the glass 20 and the wafer 10 have difference curvatures when they are hardened. As a result, the cell gap does not have uniformity. Also, even if the uniformity of the gap is maintained right after the manufacturing process, the cell gap may be deformed by the heat generated during the service of the product, which also causes the image quality deterioration.
Accordingly, in the manufacturing of an LCOS display device, the placement of the conventional spacers 25 in the LCOS display device is necessary to solve the non-uniformity problem of the cell gap. However, the placement of the spacers 25 itself is sometimes impossible in certain kinds of crystal liquid materials. For example, the placement of the spacers 25 is possible in the TN (Twist Nematic) method, whereas the crystal liquid materials used in the VA (Vertical Alignment), although having a very glaring contrast, has a defective orientation when the spacers are placed therein.
Therefore, deterioration of the image quality is caused due to it being impossible to place the spacers during the manufacture of an LCOS display device, and therefore, there is a demand for the substitution for the conventional spacers.